Rough Roving: Curiosity's Wheels Show Damage

astroengine writes "In a recent batch of images beamed back to Earth from Mars rover Curiosity's MAHLI camera, obvious signs of wear and tear could be seen in the 'skin' of the robot's wheels. Considering Curiosity is only 281 sols (Mars days) into its mission and roved less than a kilometer after landing, surely this doesn't bode well? Fortunately, there's good news. 'The wear in the wheels is expected,' Matt Heverly, lead rover driver for the MSL mission at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News. 'We will continue to characterize the wheels both on Mars and in the Marsyard, but we don't expect the wear to impact our ability to get to Mt. Sharp.'"

Comments

Fun fact

[funky49]

There's a fun fact about the wheels of Curiosity. They spell out "JPL" in Morse Code in the sand of Mars. :)

Re:Fun fact

[Taco Cowboy]

I learn something new today. Thanks !

BTW, looking at the picture ( @ http://upload.wikimedia.org/wikipedia/commons/3/37/Curiosity_wheel_pattern_morse_code.png ) I am totally surprised at the sheer thinness of Curiosity's wheel !

How can they expect Curiosity to last long with such thin wheel ??

Re:Fun fact

[Charliemopps]

Because the project managers were given criteria. 1. it had to last for X months... 2. it had to be under X kilos. Which do you think was a harder goal? I think it's pretty obvious. Making things that last forever is easy. Making things that are light is easy. Making things that are both? Little bit more difficult.

Re:Fun fact

[kuiken]

The sections where added to be used to to visually measure the precise distance between drives.

source:
http://www.nasa.gov/mission_pages/msl/news/msl20120829f.html

Wheel wearing

[girlintraining]

These wheels aren't like your normal car wheels. The very thin atmosphere means that the soil is more like lunar soil than Earth soil. Atmospheric erosion tends to smooth out sand particulate so it has a rounder shape -- it is less sharp. Lunar soil is incredibly corrosive. Think of all the problems our troops had operating in Iraq with their equipment, now multiply that by a hundred. It's like walking on microscopic needles. Martian soil isn't quite as bad, thanks to having had an atmosphere at one point, and retains a minimal one now, but it's still inhospitable.

The rover was designed with multiple wheel-sets to operate independently, and the wheels themselves designed to wear somewhat more gracefully in the face of these obstacles. But yes, they're going to look ugly fast.

Re:Wheel wearing

[Anonymous Coward]

I think you are confusing abrasive with corrosive.

Re:Wheel wearing

[Max_W]

Add to this radiation. There is no magnetic field around Mars.

Spirit and Opportunity set unrealistic expectation

[Anonymous Coward]

Since those 2 rovers outlasted their expected mission life by a factor of 20ish, everyone now expects every science mission to do similar. When they last for the amount of time they were engineered for people are disappointed. That's the danger in overachieving and the reason people feel compelled to use their full budget each year - if they're frugal for a year people expect that they'll be able to do the same every year and cut the budget. Some aspects of human nature stink.

Re:Rough Landing

[camperdave]

If it was from the landing, it would have been noticed long before now. Curiosity went through a rigorous self check before it started on its primary mission of exploring the planet.

Glad it's expected.

[sconeu]

Otherwise, it's a hell of a long wait for the AAA. And who's going to to stand there next to the rover with their card?

Mass and Weight are different

[Excelcia]

From the article:

“We have the same wheels on our Scarecrow test rover, which weighs the same on Earth as Curiosity weighs on Mars,” Heverly added. “We have driven Scarecrow about 12 kilometers (7.5 miles) in the Marsyard over rocks and slopes much harsher than we expect for Curiosity. There are some dents and holes in these wheels, but the rover is still performing well.”

This sounds an idea from the same people that brought us the Mars Climate Orbiter crater.

The problem with this is that Curiosity weighs 342kg but masses 900kg. Scarecrow weighs and masses 342kg. Whatever Curiosity weighs, it hitting a rock at 1m/s is still 900 newtons of force. Scarecrow hitting a rock at 1m/s is 342 newtons. The fact it drove 12km and has serviceable wheels does not make me feel better.

Re:Mass and Weight are different

[TrekkieGod]

Whatever Curiosity weighs, it hitting a rock at 1m/s is still 900 newtons of force. Scarecrow hitting a rock at 1m/s is 342 newtons.

Stop accusing NASA scientists of not understanding their job when you don't remember basic physics.

F = m * a, not F = m * v. In this case a is the acceleration due to gravity. In addition, mass is measured in kg, weight is measured in Newtons, because weight is a force. The newtons are exactly the same between those two rovers.

Re:Mass and Weight are different

[complete loony]

F = m * a

Look it's right there, force equals *mass* times acceleration. On earth, Scarecrow is 342kg * 1g when stationary. On mars Curiosity is 900kg * 1mars-g *when stationary*. Sure the vertical force on the tires is the same when standing still, but what about the force required to stop 342kg vs 900kg of inertia if you hit a large pointy rock at 1m/s?. In this case, with the same initial velocity, the acceleration would be the same but the force experienced by Curiosity's tires would be ~3x larger (ignoring any shock absorption).

Re:Mass and Weight are different

[TrekkieGod]

Curiosity wheel encounters rock. Wheel exerts force to lift itself over rock. To do this, wheel must lift all of Curiosity. Curiosity masses 900kg. Object at rest tends to stay at rest. Curiosity tends to stay at rest. Curiosity wheel has much inertia to overcome to make Curiosity start moving up and then over rock.

You're still incorrect. Yes, the 900 kg rover has more inertia. Inertia doesn't matter as far as how much force is being applied to the wheels, though. It means the same force will be applied as to the equivalent lower mass rover on Earth, and because of its higher inertia the rover on mars will accelerate slower while that same force is being applied.

Write the force diagram yourself. Think about what it takes to move a vehicle on wheels. How much traction do you get? the coefficient of traction times the weight . Because the only thing keeping that 900 kg vehicle on the ground to interact with those rocks as the wheel spins is the force of gravity of mars. Every action has an equal and opposite reaction: those rocks can't exert more force on the rover than the rover can exert on them, and those 900 kg are only worth on mars what the 342 kg one is here. The inertia only matters when it comes to acceleration, not when it comes to the force the wheels experience.

Now, it's entirely possible that the mars terrain is more abrasive than they expected, and it doesn't match their equivalent here on Earth. I'm willing to bet they thought about that and built in some safety margins. I'm also willing to bet they've been looking at the damage to the wheels and have by now gone through the numbers and figured out if it's going to be a problem or not. Based on what they've said, it's not going to be a problem.

Re:Mass and Weight are different

[TrekkieGod]

Sure the vertical force on the tires is the same when standing still, but what about the force required to stop 342kg vs 900kg of inertia if you hit a large pointy rock at 1m/s?

Who says the rover is stopping when it encounters a rock? Either the 342 kg one or the 900 kg one? The same amount of force will stop both, but the force will need to be applied for longer in the 900 kg rover. The term you're looking for is momentum, not force. The 900kg, assuming it's moving at the same speed as the 342 kg one, has more momentum.

Mass would certainly matter if they crashed the rover and transferred all that momentum, assuming they were moving at the same speed as the equivalent one is here (which I'm not convinced is a good assumption. They could have increased the driving speed of their Earth-equivalent in order to make momentum equal too). Either way, the reason this would matter, is because if you crash the rover against a wall, it would stop at the same rate as crashing the lighter rover into a wall on earth. In order to stop it at the same rate, more force needs to be applied. More importantly, the rover had 1/2 * m * v^2 of energy, which all had to be dissipated somewhere when v went to zero. But this isn't what is happening when the rover is driving around. The traction it gets is equal to the coefficient of traction times weight, the forces are exactly the same, and the rover merely decelerates slower when it encounters resistance, making the amount of energy dissipated the same because the difference in v is proportionally less.

Re:Mass and Weight are different

[hankwang]

force equals *mass* times acceleration. ... the acceleration would be the same but the force experienced by Curiosity's tires would be ~3x larger (ignoring any shock absorption).

You do have a point -- I don't agree with some of the other responders who talk about traction forces being smaller as well. Just to make it clear: what you say applies to a cart on wheels, having constant horizontal velocity and approaching a bump in an otherwise flat surface. A larger mass of the cart will result in a larger force at the wheels the moment the cart hits the bump, regardless of gravity.

However, this force is roughly F = m v^2/L, where v is the horizontal velocity of the cart, L is the travel of the suspension, and m is the moving mass. The moving mass can be just a single wheel; in that case L is the leeway in the tire rubber (less than a millimeter), or m can refer to the entire car, with L the travel of the wheel suspension.

Now, the issue of inertia is only relevant if the instantaneous extra force is larger than the gravitational force. Given that this Mars Rover has a maximum speed of 0.025 m/s, the maximum inertia-driven acceleration is about 1 m/s^2, even assuming only 0.5 mm of suspension travel. This is much less than the gravitational acceleration (10 m/s^2); therefore inertia does not make a significant difference in the wear on the wheels.

Re:Mass and Weight are different

[necro81]

The problem with this is that Curiosity weighs 342kg but masses 900kg. Scarecrow weighs and masses 342kg. Whatever Curiosity weighs, it hitting a rock at 1m/s is still 900 newtons of force. Scarecrow hitting a rock at 1m/s is 342 newtons. The fact it drove 12km and has serviceable wheels does not make me feel better.

That distinction is really only revelant in the case of dynamic loading: hitting things at speed, rapid straightline accerelation, or quick turning. Whether Curiosity on Mars or Scarecrow on Earth, there really isn't any of that going on. For the most part, the only loading going on is the static loading due to gravity, which they have accounted for.

You talk about hitting a rock at 1 m/s, but Curiosity's top speed on hard, flat ground is 1.5 in/sec, or about 4 cm/sec (0.14 km/h). That's only 1/25th the speed you use in your bungled calculations; 1/625 as much kinetic energy. At that slow crawl of a speed, the difference in dynamic loading between Curiosity and Scarecrow is negligible. In any event, the rover is by an large prevented from hitting anything by the route planners here on Earth and its autonomous hazard avoidance algorithms.

In other words: I trust the engineers at JPL to understand these things and account for them better than a random user on /.

Re:Mass and Weight are different

[necro81]

Sure the vertical force on the tires is the same when standing still, but what about the force required to stop 342kg vs 900kg of inertia if you hit a large pointy rock at 1m/s?

That's what's referred to as dynamic acceleration. Rolling along at 1 m/s and then coming to a sudden stop by running into a rock would indeed produce some big accelerations, and the difference in mass between Curiosity and Scarecrow would be pretty significant.

However, the scenario that you and an earlier comment are talking about - coming to an abrupt, rocky stop from 1 m/s - simply does not happen. You might be envisioning Curiosity as some sort of Martian ATV bouncing off the landscape and doing power slides, but the reality is far more prosaic. Curiosity's top speed on flat, hard soil is about 0.04 m/s, not 1 m/s. Let's say that, for whatever reason, Curiosity came to a dead stop from it's top speed in, say, 100 ms (the suspension ensures the stop is not instantaneous). That's an acceleration of 0.4 m/s^2. The static acceleration due to gravity on Mars is 3.7 m/s^2, or about 10x that.

In other words, the dynamic loading is going to be small compared to the static loading.

What is more: Curiosity has about a dozen people planning out its path, specifically to avoid running into things. Even beyond that: Curiosity has hazard avoidance cameras and autonomous algorithms that will slow or stop it before it hits anything.

Re:Spirit and Opportunity set unrealistic expectat

[hairyfeet]

Really? I think its more a testament to just how well NASA over-engineers and builds everything for the absolute worst case scenario that we get so much extra use out of these things. I mean look at how long Voyager has lasted way the hell out in the cold depths, that's a tough built ship right there.

But this is why I've always supported the robotics space exploration programs, with our current tech "meatbags in spaaace!" really doesn't make much sense. I mean when you figure in what it would take to get a crew of five to mars and back its just insanity, I remember seeing a video where they did the math and for a 3 month stay on the ground and round trip from here to there you'd have needed a ship bigger than the empire state building just to carry all the consumables and fuel needed, I don't even want to know how much it would cost for a stay as long as these rovers have had there. At the end of the day we can just get more done with the machine than we can with the man, our tech just isn't good enough right now to make "meatbags in spaace!" viable.

Re:Spirit and Opportunity set unrealistic expectat

[SuperKendall]

I remember seeing a video where they did the math and for a 3 month stay on the ground and round trip from here to there you'd have needed a ship bigger than the empire state building

Your "math" is incredibly bad. Read any book on Mars from Zubrin and become educated.

What you are overlooking is that one human in one day could day about 100x the total research done so far by all of the rovers combined. What doesn't make sense is to continue to send very expensive robots to learn less and less... we've reached the point where we simply need to send humans to really study the place.

Re:Punctured from the inside out?

[ledow]

Not really. Work in a garage for a month, you see all kinds of weird damage come in.

And this wheel is basically a cut-open barrel. Punch it on the outside and it makes a dent on the inside. It's rolling across a rocky landscape, after being basically dropped onto the planet. It probably bumps down a lot more rocks than you realise and even more than NASA ever plan, the chances of finding a level surface to wander over that doesn't have a hidden 10cm drop onto rock for at least one of the wheels hidden behind is slim. And it weighs quite a bit. Not to mention loose things getting inside the wheels and basically being inside a small tumble-dryer.

A dent in the wheel would be the least of my worries, to be honest. And there's no way you can actually tell that the dents go from inside-out or outside-in, it's an very common optical illusion. And even if the dents go "the other way", there's no way to tell from the photos that they line up - those wheels are basically taking the shape of whatever they roll over so you might find the dent going "in" is right next to a similar bend in the metal going "out".

But never let the facts stand in the way of some mad conspiracy theory, eh?

Re:Spirit and Opportunity set unrealistic expectat

[Bongo]

So robot bodies are durable but slow, human bodies versatile but fragile...

can't we send zombies?

Re:Spirit and Opportunity set unrealistic expectat

[skegg]

I mean look at how long Voyager has lasted way the hell out in the cold depths, that's a tough built ship right there.

Dude, relax, it's just science fiction. There was no caretaker, and no one was pulled into the Delta quadrant.